1. Field of Invention
The present invention relates to an archery bow, and more particularly to a compound bow with a synchronizing pulley mechanism that couples each of two flexible limbs such that they are required to flex the same amount providing the bow with a straight line movement of the nocking point for increased accuracy.
2. Description of Prior Art
Compound bows allow the archer to draw the draw string with an increasing draw weight until a peak draw weight is attained and thereafter the draw weight drops off to a minimum draw weight referred to as a let-off of draw weight. The force-draw curve for a compound bow allows the archer to store a maximum amount of energy during the draw of the draw string and hold the draw string in a full drawn position for an extended period of time at a low draw force. To obtain a favorable force-draw curve and let-off found in compound bows typically a cam-shaped pulley such as an eccentric pulley is mounted at the end of each bow limb to provide the draw string with a lever arm that is greatest in the full drawn position. The cam-shaped pulleys also require synchronization with each other such that the lever arm that they offer to the draw string is the same throughout the launch period for each of the bow limb mounted cam-shaped. pulleys. Failure to properly synchronize the cam-shaped pulleys results in nonlinear movement of the nocking point of the draw string during arrow launch resulting in inaccuracy.
U.S. Pat. No. 3,486,495 issued to Allen describes a compound bow with a single cam-shaped pulley attached to each flexible limb. A draw string passes around each of the cam-shaped pulleys and extends to the opposite limb for attachment. Any variation in limb flexibility from one limb to the other will result in a nonlinear path for the nocking point of the draw string. Each cam must be synchronized with the other to approach a straight line motion for the nocking point.
Another approach for a compound bow is described by Kudlacek in U.S. Pat. No. 4,060,066. Kudlacek places a cam pulley means at the distal end of each flexible limb. Each cam pulley means consists of two cam-shaped pulleys, one that provides for take-up and pay-out of the draw string and the other for take-up and pay-out of a power string that extends to the opposite flexible limb for attachment. This design offers an archery bow with an excellent force-draw curve but does not address movement of the nocking point due to variations in limb flexibility. The bow designs described by Kudlacek require that the cam-shaped pulleys be synchronized between the top and bottom limbs in order to reduce any additional nocking point movement due to maladjustment of the pulleys.
Jennings describes in U.S. Pat. No. 4,562,824 a bow that has a cam pulley means attached directly to the handle-riser to provide a let-off of draw force to the draw string at full draw. Attached to each of the flexible limbs are round idler pulleys that provide passage for the draw string to the cam pulley means. The cam pulley means consists of four cam pulleys that are attached together and rotate about a single axis. Since the pulleys are attached, they cannot fall out of synchronization and hence this bow overcomes the synchronization problem associated with having a cam pulley means located on each of the flexible limbs. Rotation of the four cam pulleys in the same direction during arrow launch creates an imbalanced torque on the bow that causes movement of the bow and resulting inaccuracy.
In U.S. Pat. No. 5,368,006 McPherson describes a bow with a cam pulley means attached to only one flexible limb of the bow; the other flexible limb has a round idler pulley attached. The pulley means is made up of three cam-shaped pulleys that are rotationally attached together and provide for a let-off of draw force at full draw. Although this bow overcomes the problems associated with synchronization of cams mounted on each of two limbs, it does not address nonlinear nocking point movement due to variations in limb flexibility between the two limbs.
Miller discloses in U.S. Pat. No. 5,505,185 a bow that is similar to that described by McPherson above except that he attaches two cam pulleys to the top flexible limb and three cam pulleys to the bottom flexible limb. This bow is designed to keep the nocking point in the center between the two pivot points for the two cam pulley means. This bow does not, however, keep the nocking point on a constant line with the arrow rest located on the handle-riser. Variations in flexibility of each limb will also result in movement of the nocking point in a nonlinear manner.
Helmuth describes a rigid limb bow with flexible limb springs in U.S. Pat. No. 5,535,727. This bow has two sets of cam pulleys which are attached to the rigid limbs and are coupled together to provide linear movement of the nocking point. Two additional flexible limbs have round idler pulleys attached which provide passage for power cam cables. The draw string does not directly interface with the flexible limbs and the angle of the draw string to each of the cam pulleys positioned on the rigid arms at full draw is too sharp for practical application.
Nishioka describes in U.S. Pat. No. 4,365,611 a bow with one rigid arm and one flexible arm. The lower flexible limb has a lower pulley means attached which consists of two concentric pulleys and an eccentric pulley rotatingly fixed together. The upper rigid limb has two rotatingly joined pulleys attached. A flexible link cable is used to interface with the lower pulley means and interface with an eccentric pulley attached to the handle-riser. This bow will control nocking movement in a linear direction provided that the rigid limb does not flex. Inertial effects associated with the return of the flexible limb back to a resting position during arrow launch will cause unwanted movement of this bow and reduced accuracy.
Several other bows have been described by Hofmeister (U.S. Pat. No. 3,854,467), Trotter (U.S. Pat. No. 3,923,035), and Darlington (U.S. Pat. No. 3,987,777) that comprise cam pulley means attached to the handle-riser and round concentric pulleys positioned at the ends of each flexible limb. Hofmeister, Trotter, and Darlington have each described a cam pulley means consisting of two cam pulleys that are coupled together and would require synchronization to ensure linear nocking point movement. Let-off of draw force is provided by the cam pulley means attached to the handle-riser. The cam pulley means could not be applied to a compound bow with cam pulleys located on the flexible limbs.
Problems with nonlinear nocking point movement will result in an inaccurate arrow launch. This problem is found in compound bows with flexible limbs and more specifically in compound bows that have a cam pulley means located on each of the flexible limbs. Several prior art patents have attempted to reduce the amount of nocking point movement by combining the two cams into a single cam or by moving the cam pulley means to the handle-riser. Maintaining the two cams on each of the two flexible limbs bears the advantage that the rotational torque of the two cams balances each other. No prior art disclosure describes a suitable device for synchronizing bow limb movement such that variations in limb flexibility for a bow with cams positioned on the limbs will not result in nocking point movement.